Why do we find love only to have to let it go?

Recently this has been a point I've been thinking of alot.

And I'm wondering.
Why do we fall in love, stay in love, only for things to not work out and you have to let go of it?

Why do we as humans are capable of being in love with things or people that are not good/right for us?

Why do we only accept the love we think we deserve?

Why do we fall in love only to fall out of it again? - Or was I not in love but only thinking I was in love?